A conventional method for cleaning particles from semiconductor wafers is known as megasonic cleaning. During megasonic cleaning, a transducer oscillates between compressed and strained states at a near 1 MHz rate. The transducer is operatively coupled to a source of fluid, either a fluid filled tank, or fluid flowing through a nozzle. The megasonic oscillation output by the transducer is thereby coupled to the fluid causing pressure oscillation therein. As the pressure in the fluid oscillates between positive and negative, cavitation or bubbles form in the liquid during negative pressure and collapse or shrink during positive pressure. This bubble oscillation gently cleans the surface of the wafer.
In practice, megasonic cleaners experience a number of limitations. For instance, transducers with higher power density assure better cleaning efficiency, but generate considerable heat during operation. Accordingly, transducer cooling systems are often used to prevent degradation of adhesive material that attaches a transducer to materials that couple the transducer's acoustic power to the cleaning fluid.
Such transducer cooling systems, however, undesirably increase the cost and complexity of a megasonic cleaning system.
An alternative approach has been to employ a cycled array of multiplexed transducers in which each transducer is on only 1/Nth of the cycle time, where N is the number of transducers per cleaning vessel.
The reduction of duty cycle by a factor of N, which is usually 8 for batch processing vessels for 8 inch wafers, reduces transducer temperatures and in some cases eliminates the need for transducer cooling systems. A major problem of this approach is the often unacceptable increase in processing time by a factor of N. The increase in processing time is particularly problematic for single wafer processing, where short processing time is an important requirement.
Another problem experienced by megasonic cleaners is the shadowing of the transducer's acoustic field by the wafer carrying cassette. Conventionally, two approaches are employed to address cassette shadowing. The first approach uses wafer rocking to expose shadowed parts of the wafers to acoustic field. This approach reduces the duty cycle on the shadowed parts of the wafers and thus increases the wafer's processing time. This approach also increases system cost and complexity. The second approach uses a convex transducer or convex cylindrical lens to diverge the transducer's acoustic field through the opening in the bottom of the cassette. This approach reduces the power density at the top of the cassette and increases the processing time required to clean the wafers.
Cavitation bubbles formed in the cleaning fluid during megasonic cleaning present additional challenges. Specifically bubble implosion near the surface of a wafer helps to remove particles and thus has a positive effect on cleaning efficiency. However, bubbles in the bulk of cleaning solution (i.e., not near the wafer's surface) scatter the acoustic power and thus cause a decreasing power density along the surface of the wafer as the distance from the transducer increases.
Accordingly, a need exists for an improved method and apparatus for sonic cleaning of semiconductor wafers.